1. Field of the Invention
This invention relates to apparatus and a method for clarifying water wherein the recovery of suspended particles is effected by simultaneous flotation and filtration of the suspended particles.
2. Description of the Prior Art
The demand for potable water has increased to the extent that sources of low quality water must be used to supply the increased demand. Treatment of such low quality water is necessary in order that it be of acceptable quality in appearance and safety. The water must be clear and free from suspended particles and harmful bacteria, and organic matters. Water may be made safe by chlorination and oxidation processes, and suspended particles may be removed by flotation and filtration processes.
Both the flotation and filtration processes may use the flocculation principle in which alum or other chemicals are introduced into the water to react with dissolved chemicals in the water or other added chemicals to form flocculates or so-called "flocs." The flocs are microscopic in size and have a somewhat fluffy structure resembling a snowflake or a small tuft of wool.
In treating water to make it potable it is preferable to use a minimum of flocculation agents particularly because of the cost of the large quantities required. A longer time is required when a smaller amount of flocculation agents is used in order to allow sufficient contact time for the precipitation action to take place and form flocs. After the flocs have been formed it is important that water containing the flocs is not unduly subjected to vigorous motion as by passing the water through pumps, piping, etc. which tend to break the flocs mechanically. However, in the formation of the flocs gentle agitation is necessary to bring the reacting agents into contact to form the flocs and to bring small flocs into contact to form larger flocs or clusters thereof. Upon formation the flocs engage small particles in their fluffy structure. Flocs in water settle at a theoretical rate of about one inch per minute resulting in a clarification rate of 0.5 gallon per minute per square foot of sedimentation area.
In the flotation process of flocculation air dissolved in water is introduced into the water containing the flocs and microscopic bubbles of air form on the flocs causing the flocs to be floated upwardly at a theoretical rate of about 12 inches per minute resulting in a theoretical clarification rate of 71/2 gallons per minute per square foot of flotation area. It will be noted that the flotation process is faster than the sedimentation process.
Prior apparatus for water clarification which utilized the flotation process was described in U.S. Pat. No. 4,022,696 which issued May 10, 1977 to Milos Krofta, the inventor herein. This patent disclosed a flotation tank in which its height is low compared to its area. A wheeled carriage was supported between the upper edge of the flotation tank and a central structure. A motor drive moved the carriage in a circular path around the tank. Inlet water was piped upwardly through the central structure and dispensed into the tank by means of a radially extending perforated pipe. Prior to entry into the inlet pipe, the water was treated by the injection of air and flocculating chemicals. As the water exited through the perforated pipe expansion took place and microscopic bubbles were produced which attached themselves to suspended particles in the water and tended to float the solid particles to the surface. The earlier carriage was rotated at a rate of one revolution in 2 to 3 minutes so that the majority of the suspended particles were floated to the surface by the time a revolution of the carriage had taken place.
In the Krofta patent the floated particles combined to form a floating sludge on the surface of the water which was removed by a rotating spiral scoop and discharged into a sludge compartment in the central structure. At the same time clarified water flowed from the lower portion of the tank under the action of gravity through a discharge pipe. If further clarification was required, the discharged water was passed through a filter, the constructions of which are known in the art.